SYN-DEFORMATIONAL INFILTRATION OF SURFACE-DERIVED FLUIDS ALONG FAULT ZONES IN THE IDAHO-WYOMING SALIENT, SEVIER FOLD-THRUST BELT: CONSTRAINTS FROM PAIRED RADIOGENIC AND STABLE ISOTOPIC ANALYSIS OF AUTHIGENIC CLAYS

Evidence for surface-derived fluids is increasingly documented by clays in fault rocks. However, the nature and role of fluids in thrust fault systems remains controversial. We analyzed samples from fault zones and clay-rich cleavage seams in deformed strata across the Idaho-Wyoming salient of the Sevier fold-thrust belt. For each sample, clay mineralogy was determined by quantitative XRD analysis of multiple size fractions (<0.05, 0.05-0.1, 0.1-0.5, 0.5-1, and 1-2 μm), which were subsequently analyzed for hydrogen isotopic (δD_VSMOW) composition. Additionally, samples from key fault zones and clay seams were selected for ⁴⁰Ar/³⁹Ar clay dating. The δD values of authigenic clay in seams are ~ -80‰; δD of authigenic clays in faults are ~ -90‰. The highly negative δD values in fault zones indicate involvement of surface waters during deformation. Using geologically reasonable deformation temperature estimates, calculated δD values of infiltrating fluids range from -74 to -62‰. From the global meteoric water line, we estimate corresponding δ¹⁸O_VSMOW of -10.5 to -9‰, consistent with determinations of ancient, moderate elevation to nearshore meteoric fluid compositions. Ar dating of samples constrains the timing of surface fluid infiltration. Authigenic clays in the Home Canyon fault sample near the Meade thrust have an age of 108 Ma, consistent with synorogenic strata of Albian age. A clay seam from sheared Mississippian strata in the hanging wall of the Meade thrust has an age of 102 Ma, which overlaps with deformation-mediated clay growth during Meade thrusting. The Absaroka fault rocks are dated as 73 Ma. Coarse size fractions from a clay cleavage seam in Jurassic strata in the Crawford thrust hanging wall give an age of 113 Ma, interpreted as early contraction, and an age of 56 Ma for the fine clay fraction, interpreted as late alteration, possibly coincident with a switch to extension. The pairing of Ar dating and stable isotopic measurements of clays in deformed rock documents infiltration of surface-derived fluids that were coeval with Sevier faulting and internal deformation episodes.